An aircraft structure contains many mechanical parts assembled with one another by fishplating.
Fishplating is understood to mean the assembly of elements by means of a separate part added on and attached to these elements by bolts, rivets or other through fastenings, where such an assembly is subjected to forces and/or moments.
This is the case, in particular, when assembling certain wing surface elements of the aircraft to the fuselage centre box.
Document WO 2006/100400, filed in the applicant's name, describes a fish joint device enabling such an assembly to be produced.
As shown in FIG. 1, this fish joint device includes a T-shaped fishplate 20 and a backing plate 30, attached to two consecutive structural elements 1, 2 and positioned either side of them. It should be noted that backing plate 30 is defined in this manner for the sake of linguistic convenience, in contrast with fishplate 20, and naturally constitutes a fishplate in the broad sense.
First structural element 1 is a wing lower surface panel and second element 2 is a lower panel of the centre box. A side panel 3, also called the main rib of the centre box, is also attached to T-shaped fishplate 20.
More specifically, T-shaped fishplate 20 includes a first branch for attaching wing lower surface panel 1, a second branch 22 for attaching box lower panel 2 roughly as an extension of first branch 21 (to the dihedral angle), and a third branch for attaching side panel 3 which is inclined relative to first two branches 21, 22, and roughly perpendicular to second branch 22.
First and second attachment branches 21, 22 are attached to respective structural elements 1, 2 by through fastenings 11.
Fastenings 11 are of the bolt or rivet type, and traverse fishplate 20, structural element 1, 2 in question, and backing plate 30.
A fish joint device, which contributes to forming the junction between the wing surface and the fuselage, must have satisfactory mechanical properties.
Indeed, the fuselage and the wing surface of an aircraft are subject to very substantial mechanical forces (force of gravity, aerodynamic forces, etc.). These forces cause bending, shearing and/or torsional stresses.
This requirement is also of course found in other types of application of fish joint devices.
However, the fishplate and the backing plate contain holes through which the through fastenings pass, around which the stresses tend to concentrate.
The stress concentration zones formed in this manner around the through fastenings thus have an increased risk of incipient cracks in the area of the fishplate and of the backing plate, thus impairing the mechanical properties of the fish joint device over time.
One solution consists increasing the thickness of the fishplate and/or of the backing plate so as to improve the mechanical properties of the fish joint device.
However, this increases the mass of the fish joint device, which is contrary to the habitual requirement to reduce the mass, particularly in the aeronautical field.